1. Field of the Invention
The present invention relates to a signal coding method and apparatus in which an input signal is time axis/frequency axis converted for quantization and more particularly, to a signal coding method and apparatus which can preferably be used when coding an audio signal with a high efficiency.
2. Description of the Prior Art
Conventionally, various signal coding methods are known for carrying out a signal compression by utilizing statistical characteristics in a time region and a frequency region of an audio signal (including a sound signal and an acoustic signal) and characteristics of the human hearing sense. Such coding methods can be divided into a coding in a time region, a coding in a frequency region, an analysis-synthesis coding, and the like.
When coding an audio signal, especially an acoustic signal or a music signal, more consideration is taken on the sound quality rather than the quantization efficiency. For this, a quantization by way of scalar quantization is generally used and accordingly, the lower limit of the sign bit rate is comparatively high.
However, in a rapid progress of communication media into the multimedia system, it is required to provide a technique for reduced rate coding while suppressing deterioration of the sound quality.
To cope with this, introduction of a vector quantization is required, but in the vector quantization, is productivity of a so-called isolated spectrum, i.e., some peak portions present in an audio signal spectrum are not preferable and it is desired to improve it.